This invention relates to thermoplastic polyester film, particularly to ultralow haze polyethyleneterephthalate film having a wide variety of applications.
Polyethyleneterephthalate films are used for a host of converting, printing, coating and metallizing applications. The thermal stability, dimensional stability, chemical resistance and relative high surface energy of polyethyleneterephthalate films are beneficial for typical enduse applications. For instance, polyethyleneterephthalate films are often used as coating bases for magnetic tape, thermal transfer ribbon, packaging materials, thermal lamination and many other web converted products.
Film clarity is often a key requirement for a range of applications of polyethyleneterephthalate films, herein sometimes referred to as xe2x80x9cpolyester filmsxe2x80x9d. Labels, solar control films and other optical applications require films with very high film clarity in order to satisfy enduser expectations. Films with high haze would limit the enduser acceptance of such products. However, many solutions to film clarity requirements render the polyester films difficult to handle and process. Clear polyester films are typically produced by coating or surface treating a plain almost particle free film base. This method produces clear film, but due to the surface treatment the film""s chemical resistance and scratch resistance may be compromised rendering it unsuitable for specific applications.
Found within the art are many examples of low haze, easy handling polyester films. However, these prior art materials do not meet the requirements of ultra low haze, herein defined as about xe2x89xa60.6% haze. Such low haze numbers are required for highest performance in the optical requirements described above. Furthermore, traditional solutions to low haze polyester film formulations render the film handling properties extremely poor, often leading to converter yield losses and a host of other commercial issues. Therefore, there is a need for a high clarity polyester film with improved handling characteristics.
U.S. Pat. No. 6,180,209 describes a biaxially oriented polyester film containing inorganic and organic particle fillers. The particle size distribution of the particles are particularly specified within tight ranges. Our invention advantageously creates a co-extruded film structure with aluminum oxide particles and organic fillers of minimum sizes without the requirements for specific particle size distributions. Such particle size distributions are practically difficult to achieve.
U.S. Pat. No. 5,096,773 describes a biaxially oriented polyester film containing from 0.05 to 3 wt. % of primary aluminum oxide particles of about 5 to 40 nm in size. Furthermore, these primary particles are specified to aggregate to an average particle size of from 50 to 400 nm. Furthermore, this patent teaches the requirement of an F-5 value (force required for 5% extension) in either the machine or transverse direction of not less than about 12.0 kg/mm2. Our invention differs sharply from this prior art in that we achieve a co-extruded structure, and the minimum sizes of aluminum oxide and organic fillers required to produce a high clarity, easy handling film.
U.S. Pat. No. 5,023,291 describes a polyester composition of  greater than 80 mol % of ethylene terephthalate units or 2,6-napthalenedicarboxylate units having 40-80 equivalents/10{circumflex over ( )}6 g of terminal carboxyl groups. Further specified are the requirements of from 0.01 to 5% of aluminum oxide particles of 0.005 to 5 xcexcm particle diameter, and also having a melt resistivity of 6E10{circumflex over ( )}6 to 5E10{circumflex over ( )}9 ohm-cm. Our invention uses low particle size aluminum oxide fillers in conjunction with organic particle fillers in a co-extruded film structure without particle requirements of melt resistivity.
U.S. Pat. No. 4,820,583 describes a metallized, composite polyester film with specific requirements for surface roughness. Our invention differentiates in that we have discovered specific particle type, concentration and layer thickness ranges that produce an ultra clear and easy handling polyester film.
U.S. Pat. No. 5,718,971 describes the formulation of a low haze polyester film prepared from alumina particles treated with a silane coupling agent. Our invention differentiates from this patent in avoiding the necessity of using treated alumina particles in order to obtain sufficient polymer wetting.
U.S. Pat. No. 5,475,046 describes the use of alumina particles of specific sizes added to a reactor blend to produce PET chip. This chip, when extruded, cast into sheets and biaxially oriented produces films of haze  less than 2.0% and xcexcs/xcexcd of  less than 0.7/0.6. Our invention sharply distinguishes from this patent in that we utilize much smaller particles and achieve the advantage of lower friction as well.
U.S. Pat. No. 4,828,918 describes a co-extruded film structure with low haze and low surface roughness required for the production of easy handling ultraclear films. Our invention covers the discovery of specific requirements for particle size, concentration and layer thickness necessary to produce easy handling, ultraclear polyester films.
U.S. Pat. No. 4,092,289 describes a monolayer PET film comprising particles of a mixture of pyrogenic alumina and pyrogenic silica. Our invention has specific requirements for particle size, concentration and co-extruded layer thickness necessary to produce easy handling, ultraclear polyester films.
This invention relates to an ultralow haze thermoplastic polyester film prepared by co-extrusion of a blend of polyethyleneterephthalate with inorganic and organic fillers in at least one skin layer on a virtually particle free polyethyleneterephthalate core layer, wherein the inorganic fillers are selected from the group consisting of aluminum oxide particles, silicon dioxide and mixtures thereof, of average particle size of about 0.035 xcexcm to about 0.3 xcexcm, and wherein particles of the organic filler have a particle size of less than or equal to about 0.8 xcexcm and are present in an amount less than about 0.04% by weight, based on the weight of the polyethyleneterephthalate, the skin layer being less than about 3 xcexcm in thickness.
This invention also relates to a solar control film comprising an ultralow haze thermoplastic polyester film prepared by co-extrusion of a blend of polyethyleneterephthalate with inorganic and organic fillers in at least one skin layer on a virtually particle-free polyethyleneterephthalate core layer, wherein said inorganic fillers are selected from the group consisting of aluminum oxide particles, silicon dioxide and mixtures thereof of average particle size about 0.035 xcexcm to about 0.3 xcexcm, and wherein particles of the organic filler have a particle size of less than or equal to about 0.8 xcexcm and are present in an amount less than about 0.04% by weight, based on the weight of the polyethyleneterephthalate, the skin layer being less than about 3 xcexcm in thickness.
Further, this invention relates to a label film including a polystyrene acrylate coating laminated with an ultralow haze thermoplastic polyester film prepared by co-extrusion of a layer of a blend of polyethyleneterephthalate with inorganic fillers including aluminum oxide particles of average particle size of between about 0.035 xcexcm and about 0.3 xcexcm, and said inorganic particles including one or more polymers having an average particle sizes of less than about 0.8 and are present in an amount less than about 0.04% by weight, based on the weight of the polyethyleneterephthalate, said co-extruded layer being less than about 3 xcexcm thickness.
This invention is based upon results of exhaustive tests we have conducted. The following disclosure, in describing specific test methods and results, is not intended to define or limit the scope of the invention, which is defined in the appended claims.
Test methods:
Friction was measured with the use of a Testing Machine, Inc. slip tester (TMI-Model #32-06) using ASTM D1894-95. Polyester film samples were cut to specified sizes. One sheet of polyester was clamped, xe2x80x9cAxe2x80x9d surface up, onto a 18xe2x80x3 MD (machine direction)xc3x976xe2x80x3 TD (transverse direction) glass plate. Another piece of polyester film was mounted using double-sided tape to a 2.5xe2x80x3xc3x972.5xe2x80x3200 g sled, with the xe2x80x9cBxe2x80x9d surface down. The sled was placed on top of the glass plate and attached to the load sensing device. The sled was then dragged over the film on the glass plate at 6 in/min. The only contact during the testing was polyester film surface xe2x80x9cAxe2x80x9d rubbing against polyester film surface xe2x80x9cBxe2x80x9d. The measuring distance used to calculate the value of is was 1xe2x80x3 and 4xe2x80x3 for xcexcd.
Average surface roughness (Ra) was measured using Kosaka Laboratory Limited Model #SE-30AK and #Ay-31. The average value of the data of 10 times measurements was taken as the surface roughness of the film according to the present invention. All measurements were run at 50,000xc3x97magnification and in the transverse direction of the film. The length of the measurement was 4 mm and the cut-off value was 0.08 mm.
Haze was measured using Suga Test Instruments Co. Model #HGM-2DP, using the methods of ASTM Standard D1003.
Total luminous transmission, herein referred to as TLT, was measured on a Suga Test Instruments Co. Model #HGM-2DP, using method described in ASTM Standard D1003.
Clarity was measured on a Byk Garner Hazeguard-Plus equipment, using methods described in ASTM Standard D1003.
Cloudiness was assessed by visual inspection as follows: single sheet samples of film were viewed at a distance of approximately 1 ft. In bright sunlight or under intense light at a slight glancing angle, typically less than 15xc2x0. Cloudiness is the milkiness or translucence that appears from such a viewer angle. From this assessment a rating system was established for the film samples. A rating value of xe2x80x9cpoorxe2x80x9d (Grade 10) indicates that the sample looks visably cloudy to the viewer. The samples were then further ranked according to the perceived cloudiness.
Laminate layer and main layer thicknesses were determined based on a ratio of extruder outputs.
Organic Particles
The particles were placed on the object stage of an electron microscope without overlapping them as far as possible, and observed at a magnification of 10,000 to 100,000 times using a scanning electron microscope or transmission electron microscope. In the case of a scanning electron microscope, on the surface of a sample, a platinum film of about 200 angstroms was vapor deposited using a sputtering apparatus beforehand. From the screen or photographed image, the areas of at least 200 particles were measured to calculate the equivalent diameters, and from the area equivalent diameters the volumes of the individual particles were calculated. Based on the volumes, the volume average particle diameter was calculated. (U.S. Pat. No. 5,912,074)
Inorganic Particles
A sample slurry was added to solvent (methanol) such that the concentration of the slurry/solvent was adequate to show adequate light transmission. This solution was pipetted into the Honeywell Microtrac X100 machine. The average particle size and distribution was then measured via this machine.